16bhs wall question

[Mammoth]

Hello all. I just purchased a wolf pup 16bhs today and have a quick question. I want to put either a mirror or medicine cabinet in the bathroom. My question is how thick are the walls? I ask so I know what length screws to use so I don't pop out the outside of my new camper.

And are there any other things you guys can think of that would be easy quality of life improvements? I'm gonna add a paper towel holder by sink also.

Thanks in advance for the help.